1. Field of the Invention
The present invention relates to a wireless communication network system including mobile radios mounted on a moving body and a plurality of immobile radios which is spatially separated and arranged along a moving route of the moving body and transmits information by sequentially relaying the information, in which these radios communicate by a time division multiple access method, and more specifically, relates to a wireless communication network system that can reliably relay the information to maintain a network even in a situation in which a communication failure may be likely to occur between immobile radios arranged to be capable of wirelessly communicating with each other, and moreover, the wireless communication network system that can be realized at lower cost.
2. Description of Related Art
As this type of a conventional wireless communication network system, there is a wireless communication network system applied to control trains moving along railroad tracks, for example, as disclosed in Japanese Laid-open Patent Application Publication No. 2002-12150. The wireless communication network system as disclosed in Japanese Laid-open Patent Application Publication No. 2002-12150 is provided with: two train radios (mobile radios) mounted on front and rear parts of a train moving along a railroad track and wirelessly communicating while moving; a plurality of wayside radios (immobile radios) that is spatially separated at a predetermined distance and arranged along the railroad track and each of which wirelessly communicates with an adjacent wayside radio, to sequentially relay information to transmit the information; and one control station that is connected to one of the plurality of wayside radios by a wired cable and that controls and manages communication between the plurality of wayside radios and the train radios, each of the train radios and the wayside radios performing the communication by the time division multiple access method, in which time synchronization is performed to allow each of the train radios and the wayside radios to communicate only in an allocated time slot. The wireless communication network system having such a construction is controlled in a manner in which one transmission can be done in T/N (ms), i.e., time obtained by dividing one period T (ms) by N. N is the number of time slots.
An operation of the wireless communication network system of the time division multiple access method disclosed in Japanese Laid-open Patent Application Publication No. 2002-12150 will be briefly described. It is assumed that each wayside radio is arranged at a distance interval which allows the wayside radio to receive information transmitted by an adjacent wayside radio.
A control command with respect to a train generated in a control station SC is transmitted to a first wayside radio via the wired cable. The first wayside radio wirelessly transmits the received control command to a second wayside radio upon arrival of a time of a time slot for transmission allocated to the first wayside radio, and the second wayside radio receives the control command in the same time slot. The second wayside radio wirelessly transmits the received control command to a third wayside radio adjacent thereto upon arrival of a time of a time slot for transmission allocated to the second wayside radio. Thereafter, transmission and reception are similarly performed between the wayside radios, to sequentially relay information until the information arrives at a terminal wayside radio in a control area of this network. During this relay operation of the information, when a train is at a position in which a radio wave of a wayside radio reaches, a train radio mounted on the train receives information transmitted from the closest wayside radio, and when the received information includes a control command addressed to the train radio, the train radio receives the control command, so that an in-vehicle device controls traveling of the train according to the received control command.
Each train radio transmits information from the train upon arrival of a time of a time slot for transmission allocated to the train radio. The train information is received by a wayside radio closest to each train radio. The wayside radio, which has received the train information, transmits the received train information toward the control station upon arrival of a time of a time slot for transmission allocated to the wayside radio, so that the transmitted train information is sequentially relayed as described above in a time division manner by wayside radios existing between the transmitting wayside radio and the control station, and transmitted to the control station SC. Thus, a propagation direction of the information from the control station and that of the information from the train are opposite to each other, and accordingly, each wayside radio is allocated at least two time slots for the two directions.
In such a conventional wireless communication network system, since a propagation-type network, in which adjacent wayside radios sequentially perform wireless communication to relay information, is constructed, it may occur that, when this wireless communication network is used in a tunnel, for example, a gap between a train and the tunnel may be narrow, and accordingly, a radio wave may be attenuated as being propagated through the gap. Furthermore, when each antenna of each wayside radio is installed at a lower location than a train height, antennas of wayside radios in front of and behind a train may be blocked by the train, so that a radio wave may be unlikely to be received. When the wireless propagation path is thus inhibited and a communication failure occurs between wayside radios arranged to be capable of wirelessly communicating with each other, the wireless communication network may be disconnected at a location of the train.
In the propagation-type wireless communication network system disclosed in Japanese Laid-open Patent Application Publication No. 2002-12150, since the plurality of wayside radios and one control station are configured to be connected to each other by a wired network, such a problem, that is, the disconnection of the wireless communication network caused by a train, does not occur. However, in a case in which a control area of one wireless communication network is extremely wide, such as the train control, laying of wired communication paths for connecting the plurality of wayside radios requires capital investment relating thereto and continuous maintenance thereafter, resulting in a large burden in cost.